1. Field of the Invention
The present invention relates to a method and apparatus for via drilling and selective material removal using using an ultrafast pulse laser, and more specifically it relates to an apparatus and method for via drilling and selective material removal using an ultrafast pulse laser directly from an oscillator without an amplifier, operating in picosecond and femtosecond pulse width modes.
2. Description of the Related Art
Amplified short pulse lasers of pulse widths of 100 picosecond to 10 femtosecond are being used in general applications to overcome the problem of long pulse lasers. There are several advantages of short pulse lasers in comparison to long pulse lasers. For example, since the duration of short pulse laser is shorter than the heat dissipation time, the energy does not have the time to diffuse away and hence there is minimal or no heat affected zone and micro cracks. There is also negligible thermal conduction beyond the ablated region resulting in negligible stress or shock to the surrounding material.
Since there is minimal or no melt phase in short pulse laser processing, there is no splattering of material onto the surrounding surface. There is also no damage caused to the adjacent structure since no heat is transferred to the surrounding material. There are no undesirable changes in electrical or physical characteristic of the material surrounding the target material. There is no recast layer present along the laser cut side walls, and this is vital for semiconductor applications. Amplified short pulse lasers eliminate the need for any ancillary techniques to remove the recast material within the kerf or on the surface. The surface debris present does not bond with the substrate, and it is easily removed by conventional washing techniques.
Machined feature size can be significantly smaller than the focused laser spot size of the laser beam, and hence the feature size is not limited by the laser wavelength.
Short pulse lases can be broadly divided in to two categories. The first category is the femtosecond pulse with laser (ranging from 10 fs-1 ps), and the second category is the pico second pulse width laser (ranging from 1 ps-100 ps).
The femtosecond laser system (which is generally a Ti-sapphire laser) generally consists of a mode locked femtosecond oscillator module, which generates and delivers femtosecond laser pulse of in the order of nanojoule pulse energy and 10-200 MHz repletion rate. The low energy pulse is stretched in time prior to amplification. Generally the pulse is stretched to Pico second pulse width in a pulse stretcher module, using a dispersive optical device such as a grating. The resultant stretched beam is then amplified by several orders of magnitude in the amplifier module, which is commonly called as regenerative amplifier or optical parameter amplifier (OPA). The pump lasers generally used to pump the gain medium in the amplifier are Q-switched Neodymium-yttrium-lithium-floride (Nd-YLF) laser or Nd: YAG laser with the help of diode pump laser or flash lamp type pumping. The repletion rate of the system is determined by the repletion rate of the pump laser. Alternatively if continuous pumping is used then the repetition rate of the system is determined by the optical switching within the regenerative amplifier. The resultant amplified laser pulse is of Ps pulse width is compressed to femtosecond pulse width in a compressor module. By this means femtosecond pulse of mille joules to micro joules of pulse energy of repletion rate 300 KHz to 500 Hz and average power less than 5 W are produced.
The amplified femtosecond pulse has been used widely for micro machining applications as described in U.S. Pat. Nos. 6,720,519, 6,621,040, 6,727,458 and 6,677,552. The amplified femtosecond pulse, however, suffers from limitations, which prevents it from being employed in high volume manufacturing industrial applications. The system is relatively unstable in terms of laser power and laser pointing stability. Laser stability is very essential in obtaining uniform machining quality (ablated feature size) over the entire scan field. The average laser power is relatively low to meet the industrial throughput requirements. The Amplified femtosecond laser technology is relatively expensive, which increases manufacturing costs considerably. The down time of the system is high due to the complexity of the laser system. The laser system requires relatively large floor space. There are relatively poor feature size and depth controllability due to laser power fluctuation. Experienced and trained professionals are required for the maintenance of the system.
In contrast, an amplified picosecond laser system has a pico second oscillator, which delivers picosecond laser of nanojoules pulse energy and is amplified by a amplifier. The pump lasers generally used to pump the gain medium in the amplifier are Q-switched Neodymium-yttrium-lithium-floride (Nd-YLF) laser or Nd: YAG laser with the help of diode pump laser or flash lamp type pumping. The repletion rate of the system is determined by the repletion rate of the pump laser. Alternatively if continuous pumping is used then the repetition rate of the system is determined by the optical switching within the regenerative amplifier. The resultant amplified pulse has repletion rate ranging from 500 Hz to 300 KHz of average power 1 to 10 W. Although the amplified picosecond laser is simple and compact in comparison to the amplified femtosecond laser, it has, however, several limitations, which prevents it from being used for high volume manufacturing applications in industry.
The Amplified picosecond laser is more stable than an amplified femtosecond laser system, but it is still unstable in terms of laser power and laser pointing stability to meet the needs for industrial high volume manufacturing applications. Laser stability is very essential in obtaining uniform machining quality (sblated feature size) over the entire scan field. The Amplified picosecond femtosecond laser technology is also cheaper than amplified femtosecond laser system, but it is still expensive, which increases manufacturing costs considerably. It also has relatively poor feature size and depth controllability due to laser power fluctuation. The down time of the system is relatively high, and the laser system requires relatively large floor space. Experienced and trained professionals are required for the maintenance of the system
Femtosecond laser with very low fluency is a promising machining tool for direct ablating of sub-micron structures. Fundamental pulses emitting from oscillator can be used to create nano-features. But due to short time gap between the successive pulses, there is considerable degrading of the machining quality, which is explained below.
At the end of the irradiation of an individual laser pulse, surface temperature rises to Tmax. Due to thermal diffusion, the surface temperature decays slowly and eventually reduces to the environment temperature T0. The time span of the thermal diffusion τdiffusion can be determined by the one-dimensional homogeneous thermal diffusion equation. In the case of multi-shot ablation, if the successive pulse arrives before τdiffusion (t<τdiffusion), the uncompleted heat dissipation will enhance the environment temperature. The environment temperature after n laser shots for a pulse separation of t at a time just before the next (or (n+1)th) shot can be expressed byT0(n)=T0+nδT,
where, δT is the temperature rise due to un-dissipated heat at the end of a pulse temporal separation.
The actual surface temperature Tmax(n) after n successive pulses can be written as:Tmax(n)=T0(n)+Tmax
The enhanced surface temperature of the ablation front will cause over heating and deteriorate the quality of ablation. In the case of via drilling application, such over heating deteriorate the geometry of via, causing barrel at the bottom of the hole.
The longer the time between successive pulses, the less is the effect of the thermal coupling enhancing the surface temperature. When pulse separation t is long enough that the heat diffusion outranges the thermal coupling, the machining quality of multi-shot ablation will be as good as that of single-shot ablation.
In fact, thermal coupling effect of multi-shot ablation was observed not only for nano-second pulses but also for ultrafast laser pulses. Fuerbach [1], reported that to avoid degrading of machine precision due to heat accumulating 1 μs pulse separation should be given for femtosecond pulses ablation of glass.
If pulse to pulse separation time is less than the relaxation time/diffusion time of the ablated material, there is a cumulative heating effect as described above. By this process the subsequent pulses arrive before the sample surface dissipate the heat generated by the previous pulse and relax to the state of the underlying bulk material. These effect due to heat accumulation increases with the increase in the pulse width, say from 1 fs to 100 ps. Also machining with ultrafast pulse laser directly from oscillator, the feature quality is degraded. There are several drawbacks related to the cumulative heating effects. It is difficult use such a system for nanoscale maching applications due to heat accumulation, and hence there is broadening of the feature at the focused spot. The surrounding area will be damaged due to heat accumulation, which is not accepted in many semiconductor applications. There is more debris inside and around the ablated feature, possibly resulting in considerable post processing. A barrel shape may form at the bottom of the hole in via drilling applications. There is relatively poor quality associate with the ablated feature. Accordingly, there is a need for overcoming the effect of cumulative heating, and such a technique is disclosed in the present patent application.
Drilling Interconnect Via:
In recent years, demands for higher speed and smaller chips have resulted in more complex chips having millions of interconnections. Micro-vias are used to configure multilevel and multilayer structures and integrate the components on microprocessor, gate array, or high speed computer chip. On-chip and chip-to-chip interconnections play the most significant role in determining the size, power consumption, speed, reliability and clock frequency and yield of circuit. The solution for future IC packaging is 3D IC stacking using through chip interconnects. A 3D IC is a stack of multiple dies with many direct connections tunneling through them, dramatically reducing global interconnect lengths and increasing the number of transistors that are within one clock cycle of each other. Drilling interconnect via (in Si ICs and Si interposer) are increasingly important in various applications such as laying ground plane on the back side, provision for an optical interconnect, chip scale packaging etc. After drilling via, they are coated with a layer of insulating material before the conductive material, typically copper, is deposited to make the wire. One way of producing interconnect via is by plasma etch equipment in conjunction with photolithography process. But the technique is very expensive and very slow to meet the industrial need. The fastest growing emerging tool for micro via formation is laser drilling using solid state Nd: YAG UV laser. UV wavelength in the range of 248 to 355 nm is absorbed by most materials used in IC and semiconductor fabrication. Via of 25 μm diameter can be easily achieved with UV laser.
Interconnect vias, however, fabricated with a nanosecond pulse laser as described in patents U.S. Pat. Nos. 6,631,558, 6,706,997 etc. suffer from limitations. These limitations include micro cracks, and a recast layer along the via sidewalls. It also relatively difficult to selectively drill through a layer without damaging the underlying layer, which is demanded in most interconnect via applications. It is also relatively difficult to remove surface debris due to molten material ejection from the via hole by post process cleaning. This technique cannot generate via holes in the submicron range, which is demanded by current and future integrated circuits. It also causes damage to adjacent structure due to heat dissipation. There is relatively poor via depth control which is critical in interconnect via fabrication. There is also relatively poor repeatability of via holes in terms of diameter and depth. Lastly, there is relatively poor via shape due to laser plasma shielding